What does the word"mighty heart"refer to?

Reading Comprehension: Read the quoted parts carefully and answer the questions in English. Never did sun more beautifully steepIn his first splendor, valley, rock or hill;Never saw I, never felt, a calm so deep!The river glideth at his own sweet will:Dear God! The very houses seem asleep;And all that mighty heart is lying still!(From Wordsworth's sonnet Composed upon Westminster Bridge)
What does the word"mighty heart"refer to?
【正确答案】：

“MIGHTY HEART”REFERS TO LONDON.

【题目解析】：引号中意思指代的是伦敦。